Recently, in order to respond to a demand for an increase in speed and capacity of mobile communications systems, standardizations of E-UTRA (Evolved-Universal Terrestrial Radio Access) have been conducted in the 3GPP (3rd General Partnership Project) which is a standardization project for mobile communications systems.
In the E-UTRA, a radio base station receives control information such as CQI (Channel Quality Information) and ACK (Acknowledgement)/NACK (Negative Acknowledgement) from a radio terminal via PUCCH (Physical Uplink Control Channel) (see, Non-patent Document 1).
Here, the CQI is control information indicating a receiving quality (for example, SINR) which is measured by a radio terminal. The ACK is control information indicating that data received by a radio terminal has been successfully decoded, while the NACK is control information indicating that data received by a radio terminal has failed to be decoded. In the following description, “ACK or NACK” is referred to as “A/N” if needed. The A/N is used for data retransmission control (specifically, retransmission control in a layer 2).
When CQI transmission and A/N transmission compete with each other within the same sub-frame in PUCCH, a radio terminal performs the transmissions by multiplexing a reference signal within the sub-frame with the A/N. The reference signal is a signal sequence for a radio base station to estimate a channel characteristic between the radio base station and a radio terminal. The radio base station detects A/N multiplexed in the reference signal, removes the A/N from the reference signal depending on a result of detection, and performs channel estimation using the reference signal from which the A/N is removed. The estimated channel characteristic is used for demodulation of CQI within the sub-frame.